The present invention relates to audio preamplification and more particularly to a balanced electronic circuit for impedance-coupling an audio signal source to a differential amplifier with improved noise reduction.
In the field of audio signal processing, preamplifiers are generally utilized to raise the level of a signal generated by a microphone source while providing suitable input and output impedance so that the signal may be further processed. A primary concern in the preamplification processing of such microphone-generated signals is noise degradation of the signal, and regardless of whether the source is a low impedance (about 200.OMEGA. or less) or high impedance (about 20 K.OMEGA. or more) microphone, stringent noise requirements are imposed on the associated preamplifier circuits. The signals from low impedance microphones are significantly affected by noise due to their low output levels, while the high impedance microphones are similarly subject to noise degradation primarily because of their susceptibility to stray electromagnetic and electrostatic fields.
Various preamplifier designs have been developed for microphone applications to improve noise performance. A number of such designs include transformer inputs and have offered substantially noise-free gain. However, such transformer designs are costly, introduce certain amounts of harmonic distortion, and are very susceptible to electromagnetic interference. Preamplifier circuits without transformers have also been developed providing high common-mode rejection capabilities and good noise performance. However, these transformerless circuits have been generally complex in design and have required very close tolerances on input resistive elements to avoid degradation of the circuits common-mode rejection capability due to resistor mismatches. Furthermore, the noise performance of existing transformerless preamplifiers has not been entirely satisfactory due to the adverse effects of ever-present internal noise from the transistor devices in those preamplifiers and the thermal noise generated by the typically large input resistors of such preamplifier circuits.